1. Field of the Invention
This invention relates to a method for the recovery of extractant from the aqueous effluents of a continuously operating solvent extraction circuit.
2. Description of Related Art
The operation of a solvent extraction circuit can be exemplified by the following description of a large scale circuit using mixer-settlers for processing of copper. The 10 starting material is an aqueous feed solution obtained by leaching copper ions from ore. The aqueous feed solution is mixed in tanks with an organic solvent comprising an extractant which is dissolved in an organic diluent, e.g., a kerosene. The extractant selectively forms a metal-extractant complex with the copper ions in preference to ions of other metals. The step of forming the complex is called the extraction or loading stage of the solvent extraction process.
The outlet of the mixer continuously feeds to a large settling tank, where the organic solvent (organic phase), now containing the copper-extractant complex in solution, is separated from the depleted aqueous solution (aqueous phase). This part of the process is called phase separation. Usually, the process of extraction is repeated through a total of two or more mixer-settler stages, in order to more completely extract the desired metal.
Where two or more mixer-settler stages are employed for extraction, countercurrent flow of the aqueous feed solution and the organic phase or extractant solution is employed. In a typical 3-stage extraction system, for example, the aqueous feed solution will flow through an initial mixer-settler stage ("E.sub.1 "), subsequently through a second stage ("E.sub.2 "), and then through a final mixer-settler stage ("E.sub.3 "). The organic phase will, in turn, initially contact the aqueous feed solution in E.sub.3, encounter a subsequent contact in E.sub.2 and a final contact in E.sub.1. As a result, by the time the aqueous feed solution reaches mixer-settler stage E.sub.3, substantial amounts of copper will have been extracted from it and it will be contacting an organic phase low in copper. Correlatively, when the organic phase reaches mixer-settler E.sub.1, much of the extractant will be in the form of copper-extractant complex and the organic phase will be contacting the aqueous feed solution when it is in a condition wherein little, if any, of the dissolved copper has been extracted.
After extraction, the depleted aqueous solution (extraction effluent or raffinate) may be passed through a means for recovery of readily separated organic droplets, and is then either discharged or recirculated for further leaching. Any organic phase droplets which remain associated with the effluent exit from the system along with the aqueous phase and are lost. Even in systems where the effluent is recirculated, such as in leaching of ore to regenerate a feed solution, any organic phase associated with the effluent tends to be adsorbed irreversibly on the ore and does not return to the circuit with regenerated feed.
The loaded organic phase from extraction containing the dissolved copper-extractant complex is fed to another set of mixer-settlers, where it is mixed with an aqueous strip solution of relatively concentrated sulfuric acid. The highly acid strip solution breaks apart the copper-extractant complex and permits the purified and concentrated copper to pass to the strip aqueous phase. As in the extraction process described above, the mixture is fed to another settler tank for phase separation. This process of breaking apart the copper-extractant complex is called the stripping stage, and the stripping operation is optionally repeated in a counter-current manner through a total of two or more mixer-settler stages to more completely strip the copper from the organic phase.
From the stripping settler tank, the regenerated stripped organic phase is recycled to the extraction mixers to begin extraction again, and the strip aqueous phase is customarily fed to an electrowinning tank-house, where the copper metal values are deposited on plates by a process of electrodeposition. After electrowinning the copper values from aqueous strip solution, the solution is recycled to the stripping mixers to begin stripping again. As with the extraction effluent, any organic which is associated with the strip aqueous leaving the circuit tends to be lost. Entrained organic tends to accumulate in the electrowinning cells, where the properties of the organic can be degraded. For practical purposes, this constitutes lost organic. Furthermore, the extractant tends to accumulate at the liquid surface of the electrowinning cells, causing deterioration of the quality of the deposited copper.
A similar loss of organic from a solvent extraction circuit can take place where any aqueous phase leaves the circuit after contacting the organic phase. For purposes of this invention, such an aqueous phase exiting a solvent extraction circuit is referred to as an effluent, whether it be an extraction effluent, a strip effluent, a wash effluent, or any other exiting aqueous phase.
For the most part, the organic phase associated with the effluent is not dissolved in the aqueous phase, but consists of entrained organic, that is, suspended droplets of insoluble organic phase which did not coalesce with the bulk organic phase during phase separation. Organic losses can be exacerbated by several means: organic phase may be adsorbed on undissolved solids in the aqueous phase, often referred to as crud, and be discharged with the effluent; non-ideal flow patterns in a settler may lead to locally rapid liquid velocities, sweeping out organic droplets that otherwise would have settled and coalesced; or disturbances that perturb the organic/aqueous interface may result in organic phase being carried out with the aqueous phase.
An analytical method has been used for determining the level of entrained organic in aqueous effluent from a copper solvent extraction circuit, in which a known volume of effluent is first shaken in a separatory funnel with another known volume of a water-immiscible solvent in which the entrained organic is known to be soluble. The separated solvent phase, now containing the entrained organic, is then contacted with an excess of aqueous copper solution to load the contained extractant to its maximum loading capacity. The copper level of the loaded solvent phase can then be determined to very low levels by atomic adsorption spectroscopy, and the level of extractant can be back-calculated on the basis of known stoichiometry of copper to extractant.
For most circuits, a large portion of the cost of organic phase lost is due to the contained extractant, since the extractant is often much more expensive than the diluent. For example, in copper solvent extraction the extractant may cost 25-35 times as much per pound as the diluent. Such a circuit organic phase formulated with 20% extractant in diluent can thus cost as much as 10 times as diluent alone. In other circuits the cost of reagent relative to diluent can be much higher.
Diluent often tends to be lost from a circuit more rapidly than extractant because of evaporation. Because diluents tend to be non-polar materials of relatively low viscosity, they have higher vapor pressures than extractants, and evaporate more rapidly. Thus makeup of organic in a circuit will typically require proportionately more diluent than extractant.
Solvent Extraction, Principals and Applications to Process Metallurgy, Part II, pp. 642-650, by Ritcey & Ashbrook, cites several techniques which have been applied in the solvent extraction industry to recover entrained organic phase. Flotation involves dispersing air into the aqueous phase to generate small bubbles which adsorb the organic droplets and convey them to the surface where they coalesce. A variant of this technique dissolves air into the aqueous phase under pressure, and then suddenly releases the pressure; air bubbles nucleate on the organic droplets and carry them to the surface. However, flotation techniques have limited organic recovery capacity and require significant energy input to disperse or dissolve the air. Alternatively, aqueous effluent may be passed through a coalescence vessel containing a solid with hydrophilic surfaces. Organic droplets tend to coalesce and accumulate on the solid surface; these can then be collected by backwashing and returned to the circuit. Coalescing vessels, however, can be ineffective if the aqueous is not free of solids. Carbon adsorption can be effective in removing organic droplets by adsorption, but the capacity of adsorption is relatively low, and regeneration can be expensive. Centrifuges can remove organic entrainment effectively, but are very expensive to operate and maintain. Cyclones can also be effective, but the high liquid velocity required means increased pumping costs, and the shear forces involved in pumping can actually cause finer dispersion of the entrained organic droplets.
Thus, it would be desirable to have a method for recovering the extractant lost in solvent extraction effluents in order to reduce costs. It would be especially desirable to have a method which would recover both extractant in entrained organic as well as any extractant dissolved in the aqueous phase, and which would operate simply with a minimum energy requirement.